The invention relates to fused silica having low compaction under high energy irradiation, particularly adaptable for use in photolithography applications at wavelengths of 193 and 248 nm.
Fused silica is highly relevant to such applications as windows and mirrors used in outer space, and increasingly, it is becoming relevant to optical elements for deep ultraviolet photolithography. However, it is generally known that prolonged exposure of fused silica to intense deep ultraviolet radiation of the type utilized in photolithography leads to optical damage which is generally manifested in the form of changes in the optical and physical properties of the glass.
Laser-induced optical absorption is a commonly observed problem with fused silica. In addition to induced absorption and perhaps more importantly, there is also observed in fused silica glass a physical densification or compaction of the exposed regions of the glass when exposed to high energy irradiation. Lens elements of a stepper (for photolithographic applications) which utilize deep ultraviolet wavelengths for high resolution microcircuit fabrication may become altered due to optical modification as a consequence of prolonged exposure. Even though small changes in the optical phase front produced by the effect of exposure over the life of the lens barrel are expected, at present the maximum acceptable change is not known. What is known however, is that there is a relationship between alterations in fused silica and the ultimate effect of such changes on the wavefront. The present work is directed towards a better understanding and characterization of these relationships. Compaction or densification is most readily observed by interferometry where the alteration of the optical phase front is measured through the damaged region. Usually reported as optical path length difference, OPD, densification is measured as the product of the refractive index and the path length, in parts per million.
The question of what factors contribute to the propensity of various silica materials to optical damage when irradiated with high energy laser is not settled and several possible answers have been advanced in the literature.
In the past, it has been suggested that high OH content is desirable for low induced absorption. However, high OH fused silica is not always practical because certain applications require little or no OH, for example, waveguide applications. As a result, recently it has been suggested in co-assigned U.S. Pat. No. 5,616,159 that induced optical absorption can be significantly controlled in fused silica glass regardless of the OH content by subjecting the glass to a molecular hydrogen treatment. In that connection, it has also been disclosed in co-pending, co-assigned U.S. patent application Ser. No. 08/697,094, a low OH (less than 50 ppm) fused silica glass which is highly resistant to optical damage up to 107 pulses (350 mJ/cm2) at a laser wavelength of 248 nm.
In co-pending, co-assigned U.S. application Ser. No. 08/762,513, it was suggested that high purity fused silica glass having high resistance to laser-caused optical damage can also be produced by diffusing out of the glass, molecular oxygen.
More recently, in co-assigned, co-pending PCT patent application Ser. No. PCT/US97/11697, deposited Jul. 1, 1997, titled xe2x80x9cFused Silica Having High Resistance to Optical Damage,xe2x80x9d it was suggested that radiation-caused optical damage can be minimized or eliminated by precompacting fused silica by such processes as hot isostatic pressing and by high energy pre-exposure in order to thereby desensitize the glass to subsequent high energy irradiation during actual use.
To the best of our knowledge, until now there has been little or no discussion in the literature about the cause of the induced compaction (densification), or of how this propensity to compact can be predicted in the first instance. Accordingly, it is the object of the present invention to provide a model for predicting compaction in fused silica, as well as a method for identifying glass which will be resistant to compaction.
Briefly, the invention relates to a method for characterizing fused silica glass for use in photolithographic applications. A model is provided for predicting optical distortion of fused silica. Using the inventive model, fused silica for photolithographic applications can be selected which are characterized by the fact that the glass selected by the model undergoes little or no compaction when exposed to excimer radiation in use.
In one aspect, the invention relates to a method for producing fused silica glass stepper lens exhibiting low compaction when exposed to high intensity excimer radiation of a given dose Nl2, where N is the number of pulses, and l is the fluence per pulse, said method comprising:
(a) designing the lens by:
(1) determining the intrinsic densification, (xcex4xcfx81/xcfx81)p of a sample geometry of the fused silica;
(2) determining the optical path difference xcex4(nl) of the fused silica glass at said dose; and
(3) calculating the total densification, (xcex4xcfx81/xcfx81) of the fused silica glass from the values determined in steps (a)(1) and (2); and
(b) producing the stepper lens designed in step (a).
In another aspect, the invention relates to a method for producing fused silica glass stepper lens exhibiting low compaction when exposed to high intensity excimer radiation of a given dose Nl2, where N is the number of pulses, and l is the fluence per pulse, said method comprising:
(a) designing a lens by:
(1) selecting a sample size and geometry for the lens;
(2) determining the intrinsic densification, (xcex4xcfx81/xcfx81)p of the sample;
(3) determining the optical path difference xcex4(nl) of the lens at said dose; and
(4) calculating the total densification, (xcex4xcfx81/xcfx81) of the lens from the values determined in steps (a)(2) and (3); and
(b) producing the stepper lens designed in step (a).
In a further aspect, the invention relates to a method of determining optical path damage caused by high energy irradiation in fused silica glass by:
(a) using interferometry to determine the total optical path length change xcex4(nl), by measuring (1) physical path change of the glass due to strain, and/or (2) change in refractive index due to any density change; and/or
(b) using birefringence to map the stress distribution developed as a result of densification and using the stress measurements to characterize relative density changes across the fused silica glass.
In still another aspect, the invention relates to a method of making a fused silica stepper lens for use in a photolithographic system, said lens being resistant to laser-induced densification, and said system having a predetermined expected life at an estimated excimer laser irradiation dose, the method comprising:
(a) providing a fused silica lens blank of appropriate dimension for the photolithographic system;
(b) using a finite element elastic model, extract the intrinsic laser-induced densification, (xcex4xcfx81/xcfx81)p of the blank;
(c) using interferometry, determine the optical path difference xcex4(nl) of the fused silica glass at said dose; and
(d) calculating the expected total densification, (xcex4xcfx81/xcfx81) of the fused silica glass from the values determined in steps (b) and (c);
(e) producing the stepper lens by precompacting a fused silica blank using said the expected life dose to densify said blank by an amount equal to the calculated value of (xcex4xcfx81/xcfx81) in step (d); and
(f) producing a stepper lens from the precompacted fused silica blank of step (e).
In one particular aspect, the inventive method provides a model for identifying fused silica glass having an optical path length distortion of less than 0.05 waves/cm after exposure to a high irradiation dose of l2N=8xc3x97109, where N is the number of pulses, and (l) is the fluence per pulse.